The present embodiments relate to a transmitter device for a magnetic resonance scanner having a transmitter that is arranged in the spatial proximity of a transmission coil that is connected thereto.
Magnetic resonance scanners include at least one transmission coil for generating a B1 magnetic field. The transmitter may be composed of an analog, linear class AB transmitter with a defined output resistance (e.g., 50Ω), a coaxial cable with a defined characteristic resistance (e.g., 50Ω) for transferring the transmission power into the transmission coil, a power circulator with load resistance for conducting away from the transmitter the power reflected as a result of incorrect adaptation of the transmission coil, and of the transmission coil that generates the B1 magnetic field in the object to be examined. The transmission coil is adjusted to resonate in the center of the desired frequency band, and adapted, for example, to the 50Ω input resistance with in each case at least one longitudinal capacitor and one transverse capacitor.
The arrangement has a strongly load-dependent and frequency-dependent B1 magnetic field amplitude for a given transmission power. The transmission power is to be calibrated for the desired magnetic field amplitude in a load-dependent fashion. This process is referred to as “adjustment scan”. Operation beyond the resonant frequency brings about a reduction in the B1 magnetic field owing to the reactive incorrect adaptation to the circulator impedance. The transmission coil may therefore be adjusted to resonate to the load-dependent frequency band center before the actual measurement. The bandwidth that may be achieved therefore remains dependent on the coil quality and therefore on the object (e.g., a person) arranged in the transmission coil. The output power of the transmitter therefore has a variable effect with respect to the B1 magnetic field that is generated, which makes it necessary both to adjust and to calibrate the output power of the transmitter after any change in the load.
A further disadvantage is that the transmission coil has limits with respect to the radio frequency (RF) energy that may be transmitted into the object and average power that may be transmitted into the object. During the operation of a magnetic resonance scanner, reduced RF amplitudes and pulse duty ratios may be readily possible without further limitations or degradation. This may not be ensured by conventional class AB transmitters in which the efficiency level and the average output power decrease greatly at large pulse duty ratios. However, this may lead to relatively poor image recording. Overdimensioning of the transmitter and of the power supply thereof in order to compensate for the efficiency level is, however, not readily possible in view of the maximum output power and the effects on the object to be examined in the transmission coil.
Such a transmitter device for a magnetic resonance scanner is known, for example, from DE 101 27 266 C2. In this transmitter device, in order to partially integrate the transmitter into the transmission coil, the field-generating wire loops thereof are connected to the voltage supply by switching elements that are installed in the field-generating wire loops and are actuated with a delay. As a result, adaptation and transmission elements, including a transmission cable as well as transformation elements for impedance matching to the coils, are largely dispensed with.